1. Field of the Invention
This invention relates to methods of and apparatus for making cathode electrodes for sodium sulphur cells.
A cathode electrode structure for a sodium sulphur cell comprises a fibrous mass, typically of carbon or graphite felt material, which is impregnated initially with sulphur. Sodium sulphur cells commonly employ solid electrolyte material in tubular form which separates an anodic region containing the sodium from the cathodic region containing the fibrous material impregnated with the sulphur. This fibrous material extends between the surface of the electrolyte tube and the surface of a current collector and hence is generally of annular form. It may be on the inner surface of the electrolyte tube, between the inside surface of the tube and an axially located cylindrical current collector, or it may be between the outside surface of the electrolyte tube and an outer current collector, which is commonly a metallic housing for the cell. In each of these cases, the cathodic structure is of annular form. One convenient way of making such a cathodic structure is to compress the fibrous material in a mould, to impregnate the fibrous material with liquid sulphur either before compression or after compression in the mould, then to cool the material to solidify the sulphur, while the fibrous material is still under compression in the mould, so as to form a rigid structure which can then be inserted in the cell. When the cell is heated to the operating temperature, the sulphur melts and the compressed fibrous material will expand slightly to come into contact with the current collector and the electrolyte surface.
2. Prior Art
Britich Patent Specification No. 1472975 describes a cathode structure formed as an assemblage of part cylindrical segments which together will make up the required annulus. As described in U.S. Pat. Nos. 4,176,447 and 4,243,733, a flat sheet of fibrous material may be shaped into a number of trapezoidal section segments which can be put together to form an annular structure. These segments may be formed with webs joining adjacent segments so that the whole assembly filling an annulus is a unitary structure.
Heretofore, as described for example in the aforementioned U.S. Pat. No. 4,176,447, a heated mould has been used for compressing and shaping the material and, conveniently, the sulphur is injected into the mould after the material has been compressed therein. The mould is then cooled to solidify the sulphur so as thereby to form a rigid structure which may be removed from the mould. This successive heating and cooling of the mould inevitably results in a slow production rate. Because of a viscosity change in sulphur which occurs at 158.degree. C., it is desirable to inject the sulphur at a temperature just below this e.g. at 148.degree. C. If the temperature of the sulphur is increased above 158.degree. C., then viscosity increases rapidly. If the temperature is decreased below 158.degree. C., the viscosity increases but the rate of change of viscosity with temperature is much smaller in the region just below 158.degree. C. than just above this temperature. It has always been assumed heretofore that the mould must be kept at a high temperature for injection, since the sulphur has to be forced into a fibrous matrix and it was believed that a cold mould would chill the sulphur and prevent it from properly impregnating the fibrous matrix. For this reason, it has always been considered necessary to heat and cool the mould in each cycle of operation as described above.
In the prior technique, it has been found that repetitive operation, using cyclic heating and cooling of the mould leads, after a short time, to the articles sticking to the mould.